1. Field
This application relates generally to radiation imaging devices, and, more specifically, to photosensor pixel architectures with metal-insulator-semiconductor (MIS) photodetectors and thin film transistor (TFT) readout switches.
2. Description of the Related Art
A photosensor pixel consists of a detector and an electronic readout circuit. The photosensor pixel is operated via connection to peripheral circuits (e.g., biasing, addressing, readout and digitizer circuitries). Individual photosensor pixels can be arranged in a matrix to form an array. In imaging applications, the signal from each photosensor pixel in the array can be read and arranged (i.e., multiplexed) to generate a digital electronic image.
In existing devices, the detector portion of the photosensor pixel may be a photodetector, such as a PIN photodiode, comprising a photosensitive semiconductor material. A radiation detector, e.g., an X-ray detector, often comprises a photodetector, e.g., a PIN photodiode, optically coupled to a scintillator material. The scintillator material converts high energy radiation, such as X-rays, to lower energy photons that are suitable for detection by the photodetector, such as ultraviolet, visible, or infrared photons. The readout circuit may include a thin film transistor (TFT) switch connected to the detector that is turned off while photons are collected by the detector and turned on to read out the resulting signal.
In current medical X-ray imaging applications in particular, both the detectors and TFTs in the photosensor pixels are typically made using amorphous silicon (a-Si). The existing technology for a-Si-based large area medical imagers includes flat panels in which the photodetectors and readout TFT switches are fabricated on a glass or other type of substrate. Typically, when PIN photodiodes are used for the photodetectors, an array of TFT switches is made first, and then an array of PIN photodiodes is made on top of the TFT array.
It is desirable to fabricate the photosensor and the TFT switches in the same process to shorten the fabrication process. However, the layer structure of the photosensor is often different than the layer structure of the TFT. For example, a PIN photodetector requires two types of doped barrier layers, P and N, while TFTs have only one type of doped layer, N. Thus, the PIN and TFT cannot be formed simultaneously by the same process, which complicates fabrication.
Attempts at using types of detectors other than PIN photodiodes have been made. For example, use of a metal-insulator-semiconductor (MIS) photodetector connected to a readout TFT switch has been previously described in U.S. Pat. Nos. 6,682,960, 7,012,260, and 7,208,810. There are, however, at least two shortcomings with previous MIS designs: 1) the output signal of the photosensor pixels is represented by an induced charge at the metal layer of the MIS photodetector, while maximum absorption of incident photons occurs away from the portion of the MIS photodetector where charge storage is most efficient (i.e., the interface between the insulator and the semiconductor layers); and 2) a single layer in the fabrication process serves as both the gate insulating layer of the TFT and the insulator layer of the MIS photodetector, making both insulator layer thicknesses the same.
Thus, there remains an opportunity to improve the performance and fabrication process for MIS photodetector-based photosensor pixels.